This invention relates to processing data, and more particularly it relates to a method and apparatus for creating displays, preferably tomographic displays, of three-dimensional (3D) data to aid in visualization of aggregate attribute information. More particularly, it relates to creating tomographic displays to aid in the visualization of aggregate seismic attribute information, which information identifies changes in geology, lithology, and pore fluid content within the earth""s subsurface formations.
Numerous techniques for exploring the earth to acquire geophysical data are well known. Seismic surveys, however, are the most reliable and most definitive geophysical means of structural representation currently in use. For many years seismic exploration for oil and gas reservoirs has involved the use of a source of seismic energy and its reception by an array of seismic detectors, generally referred to as geophones. When used on land, the source of seismic energy can be a high explosive charge electrically detonated in a borehole located at a selected point on the terrain, or another energy source having capacity for delivering a series of impacts or mechanical vibrations to the earth""s surface. The acoustic waves generated in the earth by these sources are partially transmitted back from strata boundaries and reach the surface of the earth at varying time intervals, depending on the distance and the characteristics of the subsurface traversed. These returning waves are detected by the geophones, which function to transduce such acoustic waves into representative electrical analog signals. In use, an array of geophones is generally laid out along a line to form a series of observations stations within a desired locality, the source injects acoustic signals into the earth, and the detected signals are recorded for later processing using digital computers, where the analog signals are generally quantized as digital sample points, e.g., one sample every two milliseconds, such that each sample point may be operated on individually. Accordingly, seismic field records are reduced to vertical and/or horizontal cross sections which approximate subsurface features. The acoustic source and the geophone array are then moved along the line to a new position and the process repeated to provide a complete seismic survey. Three-dimensional (3D) seismic surveys involve geophones and sources laid out in generally rectangular grids covering an area of interest so as to expand area coverage and enable construction of 3D views of reflector positions over wide areas.
After exploration of an area is completed, data relating to energy detected at the plurality of geophones will have been recorded, where the geophones are located at varying distances from the shotpoints. The data is then reorganized to collect traces from data transmitted at various shotpoints and recorded at various geophone locations, where the traces are grouped such that the reflections of the group can be assumed to have been reflected from a particular depth point within the earth, i.e., a common midpoint (CMP). The individual traces are then corrected for the differing distance the seismic energy travels through the earth from the corresponding shotpoints, to the common midpoint, and upwardly to the various geophones. This step includes correction for the varying velocities through the rock layers of different types. The correction for the varying spacing of shotpoint/geophone pairs is referred to as xe2x80x9cnormal move out.xe2x80x9d After this is done the group of signals from the various midpoints are summed. Because the seismic signals are of a sinusoidal nature, the summation process serves to reduce noise in the seismic record, and thus increasing its signal-to-noise ratio. This process is referred to as the xe2x80x9cstackingxe2x80x9d of common midpoints data. As is well known to those skilled in the art, processing of seismic data may vary, but normally includes normal move out, stacking, migration and deconvolution.
Originally, seismic traces were used simply for ascertaining subterranean formation structure. However, exploration geophysicists have developed a plurality of time-series transformations to obtain a variety of characteristics that describe the seismic traces, and such characteristics have been termed xe2x80x9cinstantaneous attributesxe2x80x9d because values for the attributes are generally obtained for each time sample point in the seismic data, or within a small time window of data points. These attributes provide quantitative measures of the wavelike nature of the seismic signal traces, and may characterize changes in properties of the earths subsurface formations. Examples of instantaneous attributes include, but are not limited to, amplitude, frequency, phase, dip, dip azimuth, power, pseudo porosity, etc. Attributes may be displayed as measured values of the seismic data, or may be calculated based on the seismic data. By mapping displays of such instantaneous attributes on displays of seismic section or volume data, geophysicists have characterized and identified changes in lithology, geology, pseudo porosity and pore fluid content associated with individual reflection events in the seismic trace data. Seismic attributes are not limited to instantaneous attributes, and as used herein an attribute includes any way of characterizing a seismic trace. For example, xe2x80x9cintervalxe2x80x9d attributes, which are the attributes of seismic traces calculated within a seismic interval, are often analyzed.
The sole purpose of the above described and other data processing and measurement efforts, which are known to those skilled in the art, is to facilitate the final and most critical phase of the seismic exploration method, namely, data interpretation. This interpretation includes reduction of the data to a realistic model of the subsurface strata, and illustration of both structural configurations and geologic characteristics of subsurface volumes.
Accordingly, there is a need for seismic displays that aid in understanding and characterizing various attributes by displaying aggregate seismic attribute information in an intuitive and meaningful manner.
In addition to seismic data interpretations, other areas of data interpretation can benefit from improved methods and apparatus which aid in understanding and characterizing various attributes. One such area is medical imaging, such as the imaging of brain scans. Current imaging systems are scale dependent and, hence, it is difficult to compare different brain scan attributes because the subject of one brain scan may be larger or smaller than the subject of a comparison brain scan, such as comparing a child""s brain scan to an adult""s brain scan.
Accordingly, there is a need for imaging displays that aid in understanding and characterizing various attributes and that are scale independent.
It is an object of this invention to accumulate and display values for attributes in an intuitive and meaningful manner.
It is another object of this invention to display values for attributes in a way that allows better comparison because it is scale independent.
It is another object of this invention to accumulate and display aggregate tomographic values for seismic attributes such as: amplitude, acoustic impedance, continuity factors, pseudo porosity, etc.
It is a more specific object of this invention to accumulate attribute values along a tomographic path within a subvolume of data corresponding to a 3D figure, and to map the aggregate value of the attribute for display on the surface of the 3D figure.
It is another object to compare similar subvolume displays based on their tomographic attribute maps.
Yet another object is to identify geological and stratigraphic features based on the tomographic attribute maps of subterranean volumes.
Still another object of this invention is to characterize subvolumes by combining all values of the tomographic attribute into a single number and assigning the combined value to the center point of the subvolume.
Another object is to automate geological and stratigraphic feature identification and comparison, so as to reduce subjectivity of feature identification.
Another object of this invention is to identify and compare geological and stratigraphic features by correlating attribute maps with known templates.
Yet another object is to make scale independent feature identifications and comparisons.
According to the present invention, the foregoing and other objects and advantages are attained in a method and apparatus for extracting, mapping and displaying attribute data based on intersection geometry or selected tomographic paths within an arbitrary solid figure, 3D object or region of space (hereinafter xe2x80x9c3D volumexe2x80x9d). More particularly, the present invention utilizes such method and apparatus for extracting, mapping and displaying 3D seismic attribute data onto a computer model based on selected tomographic paths within attribute 3D volumes. This method relies on computer software and involves extracting attribute data from a attribute data volume, which is generally prestored in the computer memory, and where the extracted data corresponds to data points included in a data subvolume defined by the shape of the 3D volume. A second step calls for selecting extracted data and mapping the extracted data on the surface of the 3D volume, using an appropriate distinguishing code, such as a color code or rugosity. The coded 3D volume can be displayed on a flat surface or monitor.
In a preferred embodiment, the extracted data is selected corresponding to the bounding surface of the 3D volume and mapped onto the surface of the 3D volume.
In a more preferred embodiment, the extracted data is selected along multiple tomographic paths extending from a central or other representative point in the 3D volume to the bounding surface of the 3D volume. Next, the attribute values are accumulated along each of the multiple tomographic paths to obtain a corresponding multiple of aggregate values. The aggregate values are mapped on the surface of the 3D volume.
In accordance with another aspect of this invention, a method for automating identification of features in data involves comparing the attribute maps with preexisting template maps of specific features. This is accomplished by mathematically correlating the 3D volume attribute map with the preexisting template maps using the well known correlation methods, such as normalized cross correlation algorithm, which provides a measure of the similarity between attribute maps and templates, or an extended correlation algorithm, which provides the measure of similarity in combination with a normalized absolute amplitude difference attribute maps and templates. Other methods of correlation, such as semblance and difference can also be used. The method then rejects correlations below a selected threshold value and assigns a coded feature identification of the best fit template to the attribute map being compared. Thus, feature identification are assigned to the subvolume, and a display of a data volume with values coded for identified features is provided. For example, if the method is utilized for seismic data, the method for automating identification of geological and stratigraphic features in the seismic data involves comparing the attribute maps with preexisting template stratigraphic features. Accordingly, stratigraphic features identification, such as onlap, downlap, unconformity, etc., or geologic features, such as faults, rollover, saddle, etc. are assigned to the subvolume, and a display of a data volume with values coded for identified features is provided.
In a preferred embodiment, the 3D volume is selected from the group of 3D volumes including; a sphere, a cylinder, a cube, an orthorhombic, or other arbitrary figures. Generally, the most preferred figure is the sphere, and the presently most preferred tomographic paths are the radii of the sphere.
The method and apparatus of this invention thus displays data in a form that aids in identifying specific features of the 3D volume. For seismic data, the method and apparatus of this invention, using an aggregate value for mapping a seismic attribute on the surface of a solid figure, thus displays seismic data in a form that aids in identifying specific geological, or stratigraphic features of subterranean volumes. Also, the display is orientation independent. Color coding the attribute values creates a pattern of colors on the surface of a solid figure, such as a sphere, representing the aggregate attribute for the data within the volume of the sphere. Automated feature identification allows displaying of a series of feature spheres, which are selected throughout a data volume, and permits a user to manipulate the scene to inspect the spheres from any location and orientation. This automated feature identification allows a user to compare different orientations of a map to the preexisting template to enable the user to choose the best orientation fit. Further, this automated feature reduces the time required for identification of specific features, as well as reducing the subjectivity of making the comparisons of attribute maps with preexisting templates.
Still other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description and the drawings, wherein there is shown and described only one of several preferred embodiments of the invention. As will be realized several details of this invention are capable of modification in various obvious respects without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative, and not as restrictive in nature.